The bit width of the image signal (10 to 12 bit) in the input and image processing system is set more widely than that of final output signal (8 bit) to prevent the image quality deterioration by the cancellation of significant digits of the digital signal processing in a digital still camera and the video camera. In this case, it is necessary to convert the bit width to agree with the bit width of the output system. The bit width is converted by a fixed tone characteristic to a standard scene. Moreover, the technique that the bit width is converted to apply the tone characteristic of the scene is proposed.
For instance, JP-A 2003-69821 shows an example of main subject-conscious tone correction involving estimation of a taking situation. That publication also shows an example of prevention of artifacts such as noises by imposing some restriction on the obtained tone characteristics. On the other hand, a technique for independent tone correction of an image signal for each area is proposed as well. For instance, U.S. Pat. No. 3,465,226 discloses an example wherein an image signal is divided into areas based on texture information for adaptive tone correction of each area.
A problem with fixed tone characteristics in the prior art is that proper image signals are not obtained in unordinary situations with backlit, etc. Further, a problem with the technique of JP-A 2003-69821 is that satisfactory effects on improvements are not obtained in scenes of high contrast, because tone correction occurs in one-chip image signal vs. one tone characteristic relation. According to the technique of U.S. Pat. No. 3,465,226, although there are sufficient effects on improvements obtained even in scenes of high contrast, because tone correction occurs by independent tone characteristics for each area, yet there is no limitation to individual tone characteristics. This may cause extreme transformation, leading to another problem with the occurrence of artifacts such as increased noise components and a failure of color reproduction.
In view of such problems with the prior art as described above, the main object of the present invention is to provide an image signal processing apparatus, an image signal processing program and an image signal recording medium, wherein tone characteristics are independently calculated for each area, and restriction is independently set to tone characteristics for each area so that high-definition image signals can be generated with no or little artifacts.